Dendritic cells (DCs) and T cells from macaques are comparable to their human counterparts in phenotype, function, and how they interact with immunodeficiency viruses. In particular, the syngeneic DC-T cell milieu supports active replication of HIV-1/SIV. Virus-producing syncytia (and some single T cells) are observed in such cocultures. Syncytia containing large amounts of viral nucleic acids and proteins have also been identified within the similar DC-T cell locales in vivo. Therefore, significant replication of pathogenic HIV/SIV can occur in this naturally occurring DC-T cell setting. This SIV-macaque DC-T cell model will advance critical studies on specific features of viral pathogenesis, as well as provide a system for the development and testing of vaccine and immune-based strategies. In fact, DCs are probably involved in two arms of viral pathogenesis: (1) promoting virus replication in acute and chronic stages of infection, and (2) since DCs are such potent antigen presenting cells, inducing virus-specific immune responses. Infection of macaques with an attenuated SIV defective for Nef (SIV delta Nef) results in a low level persistent infection and provides protection from subsequent infection with pathogenic SIV. However, how this vaccine effect of SIV delta Nef is induced is not known. The hypothesis of this proposal is that in the initial stages of infection with SIV delta Nef, DCs will be critical in promoting low level virus replication and inducing SIV-specific cellular immune responses. This proposal will specifically address the following questions: (1) Can DCs transmit SIV delta Nef to resting T cells in vitro? (2) Where does SIV delta Nef first target and then persist following i.v. infection? (3) Can DCs be used to monitor the development of SIV-specific T cells in SIV delta Nef-infected animals, and later in animals challenged with virulent SIV? (4) Will DCs directly exposed to SIV delta Nef in vitro induce potent anti-SIV responses in vivo? These studies will provide a greater understanding of how attenuated SIV works as a vaccine, revealing what role DCs have in the initial stages of SIV delta Nef infection and in the induction of protective immunity. This may advance the design of vaccines, as well as provide more sensitive methods to monitor the efficacy of such vaccines.